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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 05-10-2009, 09:13 PM
StarfleetRP StarfleetRP is offline
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Default Limiting AC Current

Hi, I have been looking around for ways to limit the current flowing through a circuit to a maximum value. I have only been able to find ways of doing this for DC circuits, however this is an AC circuit. I would like to limit a 25V 4A AC transformer to only have an output of 250mA (give or take). I would appreciate any advice you have on how I could accomplish this.
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Old 05-10-2009, 10:27 PM
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The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.
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Old 05-11-2009, 04:31 AM
StarfleetRP StarfleetRP is offline
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Sorry I forgot to mention that I did find a solution using a secondary transformer, but that would not be suitable for the needs of this project. However thanks for the help so far!
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Old 05-11-2009, 05:31 AM
Suzkuz Suzkuz is offline
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A resistor?
A fuse?
An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?
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Old 05-11-2009, 09:08 AM
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Quote:
Originally Posted by studiot View Post
The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.
And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'.

A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC, but would required to measure and control both the positive and negitive portion of the sine wave. Some kind of direct coupled AB amp stage with current sensing/foldback or possibly a modified H-drive circuit with current limiting could be made to work.

Lefty
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Old 05-11-2009, 12:27 PM
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Quote:
And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'
In my bathroom there is a shaver socket, protected by just such a transformer. It has never burnt out or burnt anything else yet.
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Old 05-11-2009, 12:34 PM
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It's not practical because the technology is obsolete, but a saturable reactor would be ideal for AC current limiting. Here's the Wikipedia article - http://en.wikipedia.org/wiki/Saturable_reactor
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Old 05-11-2009, 08:45 PM
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Quote:
Originally Posted by Suzkuz View Post
A resistor?
A fuse?
An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?
Ouch! What are you thinking?
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Old 05-11-2009, 08:54 PM
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Connect a 115V light bulb in series to the primary of the transformer; choosing its wattage until the current does not exceed 250mA.
Or,
Connect a 24v light bulb in series with the 24VAC secondary;
choosing its wattage until the current does not exceed 250mA.
A 12V automotive bulb may work too.

Miguel
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Old 05-11-2009, 09:20 PM
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Quote:
Originally Posted by leftyretro View Post
A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC,.....
Lefty
I was thinking along the same lines and was going to spice it. I quickly aborted because of the OP's transformer limitations. If his load requires 25VAC and the transformer is 25VAC then any solid state solution would not deliver the full 25VAC.

StarFleet, how much voltage drop can you tolerate?
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